Numerous solid materials for optically storing information have been proposed. Typically, the solid optical information medium has at least two different optical states. Information is stored in the medium by switching small regions in a thin film of the medium from one optical state to another. Generally, the switching of optical states is stimulated by a focused beam of energy in the form of coherent (laser) or incoherent light, X-rays, an electron beam, etc. The energy beam usually produces localized heating in the medium so that a relatively small region switches its optical state by thermal stimulation. In most of the known optical data storage media, the information storage or writing process is irreversible, so that after information is once written into the memory, it cannot be erased. These known media can be used only to prepare write-once, read-only memories. Information stored in an optical memory is retrieved or read by scanning the storage medium with another beam of energy, but one of insufficient energy to write additional information into the memory. For example, a detector may be disposed on the opposite side of the medium to detect whether the reading beam is transmitted through the medium in the locations where information storage, i.e. optical switching, might have occurred. Or, the reflected beam may be sensed to determine the information content at each potential data storage point in the medium. Information is present or absent depending upon the degree of transmission, reflection, polarization, etc. of the reading beam.
Most optical storage media are based on inorganic compounds or mixtures such as chalcogenides. These inorganic materials have relatively high thermal conductivities. Because these media relatively quickly conduct heat away from a writing beam, a relatively large area must be reserved for storage of each item of information and the writing beam must supply a relatively large amount of energy to record each item. Therefore, it is desirable to use low thermal conductivity materials, such as organic compounds, for optical information storage media.
An example of an organic memory medium is disclosed in U.S. Pat. No. 3,668,663. There, certain organic materials are disclosed that may be switched between dimer and monomer states by application of energy beams of appropriate frequencies. Double carbon bonds are broken or established to change the refractive index of localized portions of the organic medium. In U.S. Pat. No. 4,443,532, another organic optical storage medium is disclosed. The optical characteristics of the media described there are changed by an electron beam that is theorized to cause non-reversible crystalline changes. It is not clear whether the reported crystalline changes are really chemical reactions induced by ions created by the electron beam or are true crystalline changes.
It is desirable that an organic optical information storage medium be erasable, i.e. permit repeated writing, reading, erasing and rewriting, for versatility, and store information without a chemical reaction taking place, for reliability. The invention solves the problems of known optical information storage media by providing high storage density, organic media in which, repeatedly, information may be stored, retrieved and erased by thermally induced, reversible changes in crystalline structure.